In this project we are investigating reaginic (homocytotropic) antibody formation to antigens of gram-negative bacteria and we are attempting to determine whether these antibodies have any role in the development of microbial infections. Several strains of E. coli, P. aeruginosa, S. typhosa and other gramnegative bacteria are being tested. Bacterial antigens responsible for induction of reaginic antibodies are being isolated and characterized. Susceptibility to infections in experimental animals (mice) in which reaginic antibodies were introduced is being examined. Finally, conditions favoring reaginic antibody production during experimental bacterial infection will be established.